Such a recombination of laser sources in pulse regime requires, compared to the continuous regime, the additional demand for the pulses to have to be not only in phase, but also synchronized. In other words, to interact, the pulses must be superposed spatially and temporally. That therefore demands knowledge of the propagation times of the pulses in each of the propagation channels of the system.
The fiber systems of coherently recombined amplifier networks (CAN systems, CAN being the acronym for “Coherent Amplifier Network”), typically comprising a multitude of fiber sections, for which it is difficult to control the length with an accuracy of less than 1 cm. The optical lengths of the different channels of the system therefore typically vary by a few cms, in other words an equivalent delay of a few tens to a few hundreds of picoseconds. These delays also vary dynamically, mainly with the heating up of the system, but also with the surrounding mechanical fluctuations. Now, to add 2 laser pulses coherently, these pulses must first of all have a maximum temporal overlap: they must be synchronized. Next, to have a maximum of intensity, the pulses must in phase or co-phased.
There are many phase measurement and co-phasing techniques described for example in the publication: “kHz closed loop interferometric technique for coherent fiber beam combining” by M. Antier et al, JSTQE 20(5). With respect to the issue of the synchronization of the pulses, the method used these days consists in manually adjusting the optical delay of each propagation channel by maximizing the nonlinear interaction signal between the pulses taken 2 by 2 in a device of FROG type as presented in the publication “using phase retrieval to measure the intensity and phase of ultrashort pulses: frequency-resolved optical gating” by R. Trebino et al, J. Opt. Soc. Am A 10. According to this device, one of the pulses passes through a variable delay line, and the propagation paths of the 2 pulses meet in a nonlinear crystal. The delay line is adjusted to maximize the signal generated by nonlinear interaction between the pulses. The value of the delay line then gives the delay between the pulses. This method makes it possible to effectively realign the relative delays of each pulse relative to the others, very reliably and accurately (to within a femtosecond), but it can not be implemented collectively for a large number of pulses, even a number greater than only a few tens.
The technical issue that is sought to be resolved is therefore how to measure these delays with a large number of channels, with implementation that is robust, reliable and inexpensive, this measurement being able to be implemented in a dynamic feedback loop. This measurement of the delays must moreover be compatible with the optical phase measurement and control architectures necessary to the phase-locking of the optical channels of the source.
The proposed invention consists in exploiting, in an original manner, collective architectures for measuring optical phase by inference with an external reference. The mapping of delays between the propagation channels of the system is then obtained by placing a variable delay line on the reference channel, then by scanning this delay while analyzing the interference figure produced on the detector. The presence of fringes with strong contrast indicates the synchronization of a pulse with the reference. In the case of spatial inference fringes, the spatial position on the detector makes it possible to identify the channel concerned, and in the case of temporal interference fringes, the frequency of the fringes makes it possible to identify the channel. The complete scanning of the delay over the reference ultimately gives the complete mapping of the system and makes it possible to synchronize all of the pulses by correcting the delay on each channel, by the measured value.